PARTICLE FLUXES AND ECOSYSTEM RESPONSE ON A CONTINENTAL-MARGIN - THE 1985-1988 MEDITERRANEAN ECOMARGE EXPERIMENT

The first experiment of the ECOMARGE programme (ECOsystemes de MARGE continentale) was initiated in 1983-1984, in the Gulf of Lions (northwestern Mediterranean Sea). The objectives of the ECOMARGE-I experiment were: to quantify the transfer of particulate matter, in general, and organic carbon, in particular, from its introduction to and formation in the waters of the continental shelf-to its consumption or sedimentation on the shelf or its transfer to the slope and deep sea; and to understand the processes involved in that transfer, consumption and sedimentation together with their variability in space and time. The results of that experiment, from 1983 to 1988, are presented in this Special Issue. The highlights of the results are summarised in this paper. These results indicate that, of the particles formed in the waters of the continental shelf and those introduced by rivers, some are deposited as sediments on the shelf. A portion is transported offshore, however, to the slope and deep sea. The Rhone River, in the northeastern part of the study area, is the major source of continental material; this is transported to sea in a benthic nepheloid layer and, mostly, alongshore to the southwest. Here, it largely leaves the shelf through the canyons, especially the Lacaze-Duthiers Canyon. In the offshore waters, particle concentrations and distributions show surficial, intermediate and benthic nepheloid layers. These turbid structures increase towards the southwest, corresponding to the seaward shift of the front between the coastal waters and the Liguro-Provencal cyclonic gyre, a major forcing function in the Gulf of Lions. Considering the source and fate of particles (largely biogenic from the euphotic zone and abiogenic from deeper waters) a layered system is described, which is emphasized by the concentrations of natural and artificial elements and compounds. Of the flux of particles to the Lacaze-Duthiers Canyon, on a decadal scale, about 30% (as a minimum) is estimated to be stored as sediment; the remainder is transported down-canyon, towards the deep sea. The temporal variability of processes affecting this net seaward transport, of both biogenic and abiogenic material, is from hours, days to seasonal, and probably interannual, time scales. The response of the system to these variations is rapid, with pulses of increased discharge of particles from the adjacent shelf being detected in sediment traps in the Lacaze-Duthiers Canyon in less than 16 days (the temporal resolution of the traps). Based upon the study of tracers of particulate matter and environmental factors (i.e. river discharge and climatic conditions), it appears that the contribution from the Rhone River and its adjacent area is maximal during the winter; at this time, the flow of the Liguro-Provencal Current also increases. In contrast, the maximum relative contribution of the adjacent southwesterly area to the flux in the Lacaze-Duthiers Canyon occurs in summer, during storm events. Flux budgets of the four major constituents are presented, which demonstrate the advective transfer model; this implies decreasing recycling processes in the water column, from particulate organic carbon (POC), to opal, carbonate, and silicates. The same sequence is seen across the sediment-water interface in terms of flux to the bottom, which is stored in the recent sediments [from the more labile POC (15 mg m-2 day-1), through opal (60 mg m-2 day-1), to carbonate (600 mg m-2 day-1) and silicates (1400 mg m-2 day-1)]. The response of the benthic ecosystem is studied, by examining the distribution and temporal variability of meiofauna abundances and the reproductive biology of echinoderms. The quantitative distribution of meiofauna along the margin of the Gulf of Lions reflects clearly the distribution of "food indicators", organic carbon and nitrogen contents, chloroplastic pigment levels and bacterial numbers. The data suggest the existence of two different ecosystems: the shelf and slope-canyon benthos (< 1700 m), influenced strongly by the relatively high coastal productivity; and the lower slope and basin, where meiobenthos densities decrease substantially. The meiofaunal densities increase by a factor of 2, from summer to autumn, due to the seasonal increase in the advective input of fresh organic matter. Concerning the possible response of organisms to ecological variations on the continental margin, the reproductive biology of two species of echinoderms (living over the depth range of 60-1000 m) is examined. One of the main factors responsible for the cyclic activity could be the increase of organic carbon and amino-acid fluxes in winter, which are synchronous with gametogenesis.